A robot system is known in which various kinds of operations (hereinafter referred to as tasks) are performed with a plurality of robots. Japanese laid-open patent application publication 2003-291083 discloses a robot apparatus in which a task is performed by a plurality of robot apparatuses (paragraphs 0016 to 0045, FIGS. 1-5). This robot apparatus includes a task execution judging section that determines whether a task can be accomplished. If the task execution judging section determines that the task cannot be accomplished, the robot apparatus searches another robot that can execute the task and commands the robot to execute the task substantially at the same time.